This invention relates to an apparatus for the separation of finely divided clay particles from aqueous suspensions thereof and, more particularly, to an electroflocculation cell wherein clay particle separation is effected with minimal contamination of the separated particles by the cell anode.
Clays such as kaolin, bentonite and the like are widely used as absorbents, pigments, catalysts, paper fillers and the like. These clays generally occur in geological deposits as mixtures with inert foreign materials, and it is necessary to separate the clay material before the clay is utilized in commercial applications.
In effecting the separation, the crude clay is dispersed in water with the aid of dispersing chemicals such as sodium silicate, sodium hexametaphosphate, sodium tripolyphosphate and tetrasodium pyrophosphate and the clay slurry is blunged, degritted, classified and leached to effect removal of the undesired foreign materials. Following these steps, the clay is filtered to remove the dispersing chemicals and to produce a solid filter cake containing 50% to 60% clay. This solids cake is then redispersed and spray dried or otherwise dried in the flocculated state. For various economic reasons, it is desirable in the clay industry to ship slurries containing 70% solids. Therefore, it is conventional in the clay art to add approximately 35% to 50% spray dried clay to a 50% to 60% solids redispersed filter cake to yield a 70% solids slurry. The addition of spray dried clay to clay filter cake to prepare a 70% solids slurry for shipment adds significantly to the cost of the shipped slurry due to the relatively high costs of preparing the spray dried clay. The art, therefore, has been continually seeking to effect methods whereby clay filter cakes containing 70% clay can be directly obtained from clay suspensions without the addition of spray dried clay.
Among the methods which the art has investigated in its attempt to obtain a more concentrated, i.e., 70% solids clay cake, has been the use of electrokinetic phenomenon such as electrophoresis and electroosmosis.
Most solid materials when suspended as fine particles in a liquid acquire an electric charge. By applying a DC (Direct Current) electric field between two electrodes immersed in the particle suspension, the particles are caused to travel toward one of the electrodes and form a deposit thereon. This travelling of solid particles through a liquid due to the application of DC is referred to in the art as electrophoresis. When under the influence of a direct current potential, water or other liquid medium is caused to migrate through a stationary porous diaphragm toward a charged electrode. This phenomenon is referred to in the art as electroosmosis. Both electrophoresis and electroosmosis have been applied to the separation of clays from aqueous suspensions thereof. When applied to aqueous clay suspensions, electrophoresis is generally used to effect the deposition of the suspended clay material on a charged electrode, whereas electroosmosis functions as an aid in consolidating and concentrating the electrophoretically deposited clay by removal of the entrained water from the deposit.
In copending application Ser. No. 532,660 there is disclosed an electrokinetic cell for the separation of clay suspensions, the cell being provided with an anode and cathode separated by a semi-permeable membrane impermeable to clay particles which divides the cell into anode and cathode compartments. Upon application of direct current to the anode and cathode, clay particles in a suspension fed to the anode compartment deposit electrophoretically on the anode. A portion of the water contained in the suspension and entrained in the deposit simultaneously migrates electroosmotically therefrom and through the membrane where it is collected in the cathode compartment and thereafter removed from the cell.
One disadvantage that has been encountered in the operation of this cell is that the materials from which the anode is constructed undergo corrosion and/or erosion during operation of the cell which causes foreign metal ions (e.g. lead ions) or particulate matters (such as carbon) to contaminate and reduce the brightness or otherwise discolor the clay particles deposited thereon, thereby rendering the clay deposit unacceptable for many of its intended commercial uses.